Calorimetric gas sensors are potentially useful for measuring on-board the concentration of combustibles in automotive exhaust systems and for correlating increased levels of combustibles with the deterioration of the hydrocarbon (HC) conversion efficiency of an automotive three-way catalyst (TWC).
Si-based calorimetric devices, such as the one described in U.S. Pat. No. 5,451,371, have been shown to have the ability to detect a minimum concentration, called the detection limit, of the order of 10 ppm of C.sub.1 when the flow and the temperature of the gas stream are well controlled. This detection limit is sufficiently low to allow the measurement of low hydrocarbon concentration levels in the exhaust gas of newer automotive vehicles. However, in many practical applications, including exhaust gas sensing, the device is exposed to large temperature fluctuations, especially in a high velocity, turbulent flow environment. Such temperature fluctuations produce an increase in the device noise because they cannot be completely compensated for by the differential nature of the device, and, as a result, the device detection limit deteriorates. In addition, the zero-offset of differential calorimeters that are based on the difference between two resistive temperature detectors (RTDs), one being a reference element and the other a sensing element, needs to be accurately trimmed to achieve 10 ppm of C.sub.1 calibration accuracy. Furthermore, any drift in the zero-offset during the life of the device reduces the long term accuracy of the sensor. There is thus a need to improve the accuracy and the detection limit of calorimetric gas sensors for measuring exhaust gas constituents.